Figure 29 illustrates the wiring diagram for a four-lead thermometer used with a potentiometer. The current adjusted to a suitable value by the rheostat n flows through a resistance standard R and the thermometer coil r4. The potential drops across R and r4 are measured by the potentiometer. Whence from Ohm's law, e/R = <?'A*4 where e and e' are the potential drops across R and r4 respectively. Thus knowing e, e/ and R, the coil resistance r4 is readily obtained. The resistance R maybe made adjustable so that by varying R until e = e ' the dial setting gives directly the value of r4 = R. This method is useful in precise laboratory work.
Deflection Galvanometer Method.—The bridge methods so far described require the adjustment of the bridge until the galvanometer shows zero deflection. The temperature is then read from the setting of the slide wire. It is possible,
FIG. 29.—Four-lead potentiometer circuit.
FIG. 30.—Unbalanced Engelhard Wheatstone bridge.
however, to use an unbalanced bridge with all resistances fixed, except that of the thermometer, which depends upon its temperature, and the temperature is obtained from the scale reading of the galvanometer. Figure 30 shows the wiring ^ diagram for the Engelhard unbalanced Wheatstone bridge. With the switch S thrown to the right we have the simple Wheatstone bridge except that all the resistances n, r2, and r3 are fixed. At a certain temperature of the thermometer, for example -10°C., its resistance is such that the bridge is balanced and r3/ri = r4/r2. If the temperature of the thermometer increases, its resistance increases and the bridge is no longer balanced. Thus a current flows through the galvanometer producing a deflection. The deflection of the galvanometer in general depends upon two factors, the temperature of the thermometer and the current flowing from the battery B. This current is adjusted to a definite value in the following manner. The switch S is thrown to the left as illustrated, thus replacing the thermometer by a fixed resistance of such magnitude that a suitable value of the current from B deflects the galvanometer to its full-scale deflection or to a normal point marked on the scale. The resistance R is adjusted until the galvanometer deflects the proper amount.